1. Field of the Invention
The present invention relates to a calibration disk used for an optical disk inspection unit to correct the measurement values of the mechanical characteristics, such as plane deflection, eccentricity, warp angle and roundness, of optical disks including write-once disks, magneto-optical disks, and compact disks (CD).
2. Prior Art
The control accuracy of the focusing and tracking actuators of optical recording/reproducing units, such as disk players and disk recorders, is greatly dependent on the plane deflection, eccentricity and other mechanical characteristics of an optical disk. It is thus necessary to inspect that such mechanical characteristics conform to the specified standard during the production of the optical disks. By this inspection, interchangeability of the optical disks can be guaranteed.
FIG. 8 is a general block diagram illustrating an example of an optical disk inspection unit used to measure the mechanical characteristics of the optical disks described above.
Numeral 11 designates a light source (HeNe laser). Numerals 12a to 12d designate mirrors. Numeral 13 designates a half mirror. Numerals 14a and 14b designate fixed lenses. Numerals 15a and 15b designate beam splitters. Numeral 16 designates a .lambda./4 plate. Numeral 17 designates an object lens. Numeral 18 designates the actuator of the object lens 17. Numeral 19 designates a plane deflection measurement mirror. The object lens 17 is integrated with the plane deflection measurement mirror 19.
Numeral 21 designates a photodetector. Numeral 22 designates a focus-control circuit. The output terminal of the focus control circuit 22 is connected to the actuator 18. Numeral 23 designates an optical position sensor. A circuit including this optical position sensor 23 forms an axial displacement detection circuit. Numeral 24 designates an optical disk to be tested.
The functions of the optical disk inspection unit shown in FIG. 8 are described below.
The laser beam generated from the light source 11 is divided by the half mirror 13. A part of the beam enters the optical disk 24 (supported by the disk support 28 of the optical disk inspection unit) to be tested via the mirror 12b, fixed lens 14a, beam splitter 15a, .lambda./4 plate 16, mirror 12c and object lens 17.
This incident beam is reflected by the information recording plane (not shown) of the optical disk 24 to be tested. The reflected beam passes the .lambda./4 plate again and the deflection wave surface of the plate turns by .lambda./2 due to going and returning of the beam. The beam is then reflected by the beam splitter 15a and enters the photodetector 21. The photodetector 21 outputs a voltage corresponding to a focus error. The focus-control circuit 22 uses this output voltage to drive the actuator 18 so that the object lens 17 follows the recording layer deflection of the disk. The other part of the beam divided by the half mirror 13 enters the plane deflection measurement mirror 19 via the beam splitter 15b, mirror 12d and fixed lens 14b.
Since the plane deflection measurement mirror 19 follows the recording layer deflection of the disk as described above, the recording layer deflection of the disk is detected as the change of the beam spot position on the optical position sensor 23 using the beam, which enters the optical position sensor 23 via the beam splitter 15b and is reflected by the mirror 19. This change is converted into an electrical output by an axial displacement detector including the optical position sensor 23 and is used to measure the recording layer deflection of the disk.
FIG. 9 is a general block diagram illustrating another example of an optical disk inspection unit used to measure the mechanical characteristics of optical disks. This optical disk inspection unit uses a capacitance detector 27 to detect the displacement of the object lens 17 which follows the axial dynamic deflection of the disk as the change in electrostatic capacitance between a fixed electrode plate 26 and a movable electrode plate 25 installed on the object lens 17 and thus to measure the axial deflection of the disk.
The eccentricity of the disk can be measured by detecting the radial displacement of the object lens 17 which follows the tracks of the disk using the optical or capacitive displacement detector in the same way as mentioned above (Japanese Patent Application No. 60-65784).
The mechanical characteristics of the optical disk 24 are inspected as described above. Usually the inspection unit for measuring these mechanical characteristics needs to be corrected periodically to maintain its measurement accuracy. A conventional correction method is described below.
A fine adjustment jig, which has already been corrected for the displacement, is provided to the mechanical characteristics inspection unit. In order to correct the axial displacement detector, a mirror equivalent to the disk 24 is adjusted to slightly displace in the axial direction by using the adjustment jig. At this time, the object lens 17 is driven to follow the movement of the mirror by the focusing actuator controlled using the servo system composed of the photodetector 21 and focus-control circuit 22. The axial displacement detector generates an electrical output corresponding to a given axial displacement of the mirror. The correction of the axial displacement detector is made for varying displacements as mentioned above.
To correct the radial displacement detector installed on the object lens of the optical disk mechanical characteristics inspection unit, an optical system as shown in FIG. 10 is formed so that the position of the object lens 17 can be detected by using a beam position sensor 29 which has already been corrected. When the object lens 17 is driven forcibly in the radial direction, the radial displacement of the object lens 17 can be detected by the beam position sensor 29. The output of the radial displacement detector 30 obtained at this time is compared with the displacement of the object lens 17 obtained from the output of the beam position sensor 29 to calibrate the radial displacement detector 30.
In addition, the inspection unit needs to be designed by considering the installation position of the fine adjustment jig. As a result, the structure of the inspection unit is apt to become complicated. The inspection unit should be corrected frequently to maintain high optical disk measurement accuracy. In the case of the above-mentioned conventional correction method, the positional relationship between the plane deflection measurement mirror 19 and the displacement detection members needs to be adjusted very accurately. This correction method is complicated and requires much labor. There are many causes for errors, and high correction accuracy is not obtained. Furthermore, correction efficiency is low since it is difficult to automate the correction process.